Electrochemical cells for direct oxide reduction, and related methods

ABSTRACT

A method of direct oxide reduction includes forming a molten salt electrolyte in an electrochemical cell, disposing at least one metal oxide in the electrochemical cell, disposing a counter electrode comprising a material selected from the group consisting of osmium, ruthenium, rhodium, iridium, palladium, platinum, silver, gold, lithium iridate, lithium ruthenate, a lithium rhodate, a lithium tin oxygen compound, a lithium manganese compound, strontium ruthenium ternary compounds, calcium iridate, strontium iridate, calcium platinate, strontium platinate, magnesium ruthenate, magnesium iridate, sodium ruthenate, sodium iridate, potassium iridate, and potassium ruthenate in the electrochemical cell, and applying a current between the counter electrode and the at least one metal oxide to reduce the at least one metal oxide. Related methods of direct oxide reduction and related electrochemical cells are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.17/128,779, filed Dec. 21, 2020, pending, which is a continuation ofU.S. patent application Ser. No. 15/886,041, filed Feb. 1, 2018, nowU.S. Pat. No. 10,872,705, issued Dec. 22, 2020, the disclosure of eachof which is hereby incorporated herein in its entirety by thisreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contract NumberDE-AC07-05-ID14517 awarded by the United States Department of Energy.The government has certain rights in the invention.

TECHNICAL FIELD

Embodiments of the disclosure relate generally to electrochemical cellsfor reducing one or more oxides, and related methods. More particularly,embodiments of the disclosure relate to the direct oxide reduction (DOR)electrochemical cells for reducing one or more metal oxides at thecathode of the electrochemical cell, the electrochemical cell comprisingone or more anode materials that are substantially inert in a moltensalt electrolyte of the electrochemical cell, and related methods ofreducing metal oxides in the electrochemical cell.

BACKGROUND

Processing of spent nuclear fuels is often performed with the goal ofextracting uranium and plutonium from the spent nuclear fuels. Oneexample of processing spent nuclear fuels is the so-called “Purex”process wherein the spent nuclear fuel is dissolved in nitric acid.Insoluble solids of the spent nuclear fuel are removed from the nitricacid solution. An organic solvent (e.g., tributyl phosphate (TBP)) mixedwith a hydrocarbon solvent, such as kerosene, is used to extract theuranium and plutonium from the nitric acid solution to formUO₂(NO₃)₂.2TBP complexes and similar complexes of plutonium. Theplutonium is subsequently separated from the uranium, such as byexposure to aqueous ferrous sulphamate.

Other methods of processing used oxide nuclear fuels include dissolvingthe used oxide nuclear fuel in an electrolyte and electrowinning thedissolved materials (e.g., dissolved uranium ions) by passing a currentthrough the electrolyte including the dissolved materials. The currentforces the dissolved uranium ions to move toward the cathode, where theuranium ions are reduced and deposited on the cathode surface.

In addition to nuclear fuels, many metal oxides may be purified and/orconverted to their constituent metals in an electrochemical cell.Reduction of metal oxides in an electrochemical cell conventionallyinvolves dissolution of the metal oxide in an electrolyte.

The electrolyte of such electrochemical cells may include a molten saltelectrolyte. Conventional anode materials used in such electrochemicalcells include graphite, tin oxide, cermets, ceramic materials, andmetals such as platinum. The molten salt is often corrosive to one ormore portions of the electrochemical cell, such as the anode. Forexample, molten salts that contain lithium may degradeplatinum-containing anodes by way of forming lithium platinate(Li₂PtO₃). A portion of the lithium platinate dissolves in the moltensalt electrolyte. Over time, the anode material is thinned andeventually dissolves completely in the molten salt electrolyte orotherwise becomes unusable in the electrochemical cell.

In addition, depending on the particular configuration of theelectrochemical cell, oxygen gas may evolve at the anode. The evolvedoxygen may react with the platinum anode to form oxides of platinum(e.g., platinum (II) oxide (PtO), platinum (IV) oxide (PtO₂), Pt₃O₄)that dissolve in the molten salt electrolyte. Since the oxides ofplatinum are soluble in the molten salt electrolyte, the anode materialis dissolved in the molten salt electrolyte and, as a result, the anodematerial is thinned, further reducing the usable life of the anode. Inaddition, spent nuclear fuels include fission byproducts, which ofteninclude corrosive gases such as selenium, tellurium, and iodine. Underoperating conditions of the electrochemical cell, these corrosive gasesundesirably react with the anode to form soluble intermetalliccompounds. As the soluble intermetallic compounds form and dissolve inthe molten salt, the anode is further degraded. Over time, as moreplatinum reacts with the components of the molten salt (e.g., lithiumoxide ions, and corrosive gases), the platinum anode material isprogressively thinned and consumed in the electrochemical cell.

As another example, graphite-containing anodes may react with the oxideions in the electrochemical cell, generating carbon dioxide and carbonmonoxide, as well as carbon dusts on the surface of the molten saltelectrolyte. The carbon dust contaminates the components of theelectrochemical cell. The generation of carbon dioxide, carbon monoxide,and carbon dust consumes the graphite anode. As the anode thins and isconsumed in the electrochemical cell, long term operation of theelectrochemical cell is hindered. If sufficient portions of the anodematerial are consumed, the anode needs to be replaced for the desiredelectrolytic reduction reaction to proceed.

BRIEF SUMMARY

Embodiments disclosed herein include methods of reducing metal oxideswith a molten salt electrolyte and to related methods. For example, inaccordance with one embodiment, a method of reducing spent nuclear fuelscomprising uranium oxide comprises providing an electrochemical cellcomprising a working electrode, and a counter electrode comprising oneor more materials selected from the group consisting of osmium,ruthenium, rhodium, iridium, palladium, silver, gold, lithium iridate,lithium ruthenate, lithium rhodates, a lithium tin oxygen compound, alithium manganese compound, strontium ruthenium ternary compounds,calcium iridate, strontium iridate, calcium platinate, strontiumplatinate, magnesium ruthenate, magnesium iridate, sodium ruthenate,sodium iridate, potassium iridate, and potassium ruthenate, disposing auranium oxide material on or proximate the working electrode, exposingthe uranium oxide to a molten salt electrolyte comprising at least oneof lithium chloride, lithium oxide, calcium chloride, calcium oxide, andsodium chloride, and providing an electric current between the counterelectrode and the working electrode to reduce the uranium oxidematerial.

In additional embodiments, a method of direct oxide reduction comprisesforming a molten salt electrolyte in an electrochemical cell, disposingat least one metal oxide in the electrochemical cell, disposing acounter electrode comprising a material selected from the groupconsisting of osmium, ruthenium, rhodium, iridium, palladium, platinum,silver, gold, lithium iridate, lithium ruthenate, a lithium rhodate, alithium tin oxygen compound, a lithium manganese compound, strontiumruthenium ternary compounds, calcium iridate, strontium iridate, calciumplatinate, strontium platinate, magnesium ruthenate, magnesium iridate,sodium ruthenate, sodium iridate, potassium iridate, and potassiumruthenate in the electrochemical cell, and applying a current betweenthe counter electrode and the at least one metal oxide to reduce the atleast one metal oxide.

In further embodiments, an electrochemical cell comprises a counterelectrode comprising a material selected from the group consisting ofosmium, ruthenium, rhodium, iridium, palladium, platinum, silver, gold,lithium iridate, lithium ruthenate, a lithium rhodate, a lithium tinoxygen compound, a lithium manganese compound, strontium rutheniumternary compounds, calcium iridate, strontium iridate, calciumplatinate, strontium platinate, magnesium ruthenate, magnesium iridate,sodium ruthenate, sodium iridate, potassium iridate, and potassiumruthenate, a working electrode comprising a metal oxide to be reduced inthe electrochemical call, and a molten salt electrolyte comprising amolten salt comprising at least one of an alkali halide salt and analkaline earth metal halide salt.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic of an electrochemical cell for directoxide reduction, in accordance with embodiments of the disclosure;

FIG. 2 is a simplified cross-sectional view of an anode for theelectrochemical cell, in accordance with embodiments of the disclosure;

FIG. 3 is a simplified schematic of an electrochemical cell for formingthe anode of FIG. 2 , in accordance with embodiments of the disclosure;

FIG. 4 is a simplified flow diagram of a method of reducing one or moremetal oxides, in accordance with embodiments of the disclosure; and

FIG. 5 is a graph showing a relationship between a measured current as afunction of time in an electrochemical cell, in accordance with anembodiment of the disclosure.

DETAILED DESCRIPTION

Illustrations presented herein are not meant to be actual views of anyparticular material, component, or system, but are merely idealizedrepresentations that are employed to describe embodiments of thedisclosure.

The following description provides specific details, such as materialtypes, dimensions, and processing conditions in order to provide athorough description of embodiments of the disclosure. However, a personof ordinary skill in the art will understand that the embodiments of thedisclosure may be practiced without employing these specific details.Indeed, the embodiments of the disclosure may be practiced inconjunction with conventional fabrication techniques employed in theindustry. In addition, the description provided below does not form acomplete process flow, apparatus, system or method for reducing a metaloxide. Only those process acts and structures necessary to understandthe embodiments of the disclosure are described in detail below.Additional acts to reduce a metal oxide may be performed by conventionaltechniques. Also note, any drawings accompanying the present applicationare for illustrative purposes only, and are thus not drawn to scale.Additionally, elements common between figures may retain the samenumerical designation.

As used herein, the term “platinum group metal” (PGM) means and includesa metal including at least one of ruthenium, osmium, rhodium, iridium,palladium, and platinum.

According to embodiments described herein, an electrochemical cellcomprises a molten salt electrolyte, a counter electrode (e.g., ananode) in contact with the molten salt electrolyte, and a workingelectrode (e.g., a cathode) in contact with the molten salt electrolyte.The molten salt electrolyte includes a molten salt of an alkali metalhalide salt, an alkaline earth metal halide salt, an alkali metal oxide,an alkaline earth metal oxide, or combinations thereof. The workingelectrode may comprise a metal oxide to be reduced in theelectrochemical cell or the metal oxide to be reduced may be in directcontact with the working electrode. In some embodiments, the workingelectrode consists essentially of the metal oxide to be reduced. Themetal oxide may comprise a transition metal oxide, a lanthanide oxide,an actinide oxide, or combinations thereof. In some embodiments, themetal oxide comprises spent uranium oxide nuclear fuel (e.g., depleteduranium oxide fuel), unirradiated nuclear fuel (e.g., enriched uraniumoxide fuel), or combinations thereof. In use and operation, the metaloxide is reduced at the working electrode and oxide ions are generatedat the working electrode. The reduced metal remains at the workingelectrode and the oxide ions are dissolved in the molten saltelectrolyte. Accordingly, the metal is not substantially dissolved inthe molten salt electrolyte. Responsive to exposure to an electriccurrent between the counter electrode and the working electrode, theoxide ions move from the working electrode to the counter electrodethrough the molten salt electrolyte.

The counter electrode comprises a material that is substantially inertor otherwise not substantially consumed in the electrochemical cell. Insome embodiments, the counter electrode comprises a material that isstable in an oxidizing atmosphere at an operating temperature of theelectrochemical cell. In some embodiments, the material of the counterelectrode comprises at least one platinum group metal (PGM), at leastone precious metal (e.g., gold or silver), or a combination thereof. Byway of nonlimiting example, the counter electrode is selected from thegroup consisting of at least one of osmium, ruthenium, rhodium, iridium,palladium, platinum, silver, gold, lithium iridate (Li₂IrO₃), lithiumruthenate (Li₂RuO₃), a lithium rhodate (LiRhO₂, LiRhO₃), a lithium tinoxygen compound (e.g., Li₂SnO₃), a lithium manganese oxygen compound(e.g., Li₂MnO₃), calcium ruthenate (CaRuO₃), strontium ruthenium ternarycompounds (e.g., SrRuO₃, Sr₂RuO₃, Sr₂RuO₄), CaIrO₃, strontium iridate(e.g., SrIrO₃, SrIrO₄, Sr₂IrO₄), calcium platinate (CaPtO₃), strontiumplatinate (SrPtO₄), magnesium ruthenate (MgRuO₄), magnesium iridate(MgIrO₄ 4), sodium ruthenate (Na₂RuO₄), sodium iridate (Na₂IrO₃),potassium iridate (K₂IrO₃), and potassium ruthenate (K₂RuO₄).

In some embodiments, the counter electrode comprises a substratecomprising a different material than the at least one platinum groupmetal or the at least one precious metal and the substrate is coatedwith the material of the counter electrode. The counter electrode may besubstantially inert in the electrochemical cell. The counter electrodemay resist attack from the molten salt electrolytes, which may becorrosive at high temperatures (e.g., greater than about 600° C.,greater than about 800° C., etc.) under oxidizing conditions. Thecounter electrode may exhibit good electrical conductivity suitable foroperation in the electrochemical cell. Accordingly, the material of thecounter electrode may not be consumed during the electrochemicalreduction reaction (e.g., direct oxide reduction) and the counterelectrode may not need to be replaced as in conventional electrochemicalcells.

FIG. 1 is a simplified schematic of a system 100 including anelectrochemical cell 102 for reducing one or more metal oxides to formone or more metals. The electrochemical cell 102 may be configured as aso-called “direct oxide reduction” (DOR) electrochemical cell. In otherwords, the electrochemical cell 102 may be configured to reduce one ormore oxides.

The electrochemical cell 102 may be contained within a gas-tightenclosure 104, which may include an inlet 106 and an outlet 108. Theinlet 106 is configured for providing, for example, a gas to theenclosure 104 for maintaining a gas pressure within the enclosure 104.Gases may be removed from the enclosure 104 via the outlet 108. In someembodiments, the gas comprises an inert gas, such as argon, helium, or acombination thereof. The enclosure 104 may include a furnace or otherheating element for heating or maintaining a temperature of a moltensalt electrolyte 110 in the electrochemical cell 102. Although FIG. 1illustrates that the enclosure 104 include the inlet 106 and the outlet108, the disclosure is not so limited. In other embodiments, theenclosure 104 may be configured as a so-called “glove box” wherein theenclosure is not configured with an inlet 106 and an outlet 108 for gasflow into and out of the electrochemical cell 102 during operationthereof.

The electrochemical cell 102 may include a crucible 112 comprising ametal, glassy carbon, ceramic, a metal alloy, or another material. Insome embodiments, the crucible 112 comprises a non-metallic material,such as alumina (Al₂O₃), magnesia (MgO), glass carbon, graphite, boronnitride, another material, or combinations thereof. In otherembodiments, the crucible comprises a metal or metal alloy, such as, forexample, nickel, molybdenum, tantalum, stainless steel, alloys of nickeland copper, alloys of nickel, chromium, iron, and molybdenum, alloys ofnickel, iron, and molybdenum, and combinations thereof.

The molten salt electrolyte 110 may be disposed in the crucible 112. Theelectrochemical cell 102 may further include at least one counterelectrode 114 (which may also be referred to as an anode) and at leastone working electrode 116 (which may also be referred to as a cathode).In some embodiments, the electrochemical cell 102 further includes areference electrode 118 configured for monitoring a potential in theelectrochemical cell 102. In some embodiments, a sheath 122 is disposedaround at least a portion of one or more of the counter electrode 114,the working electrode 116, and the reference electrode 118. The sheath122 may be configured to provide electrical insulation between therespective electrodes and the crucible 112. In some embodiments, thesheath 122 comprises alumina (e.g., an alumina tube), magnesia, or acombination thereof.

The reference electrode 118 may be in electrical communication with thecounter electrode 114 and the working electrode 116 and may beconfigured to monitor the potential difference between the counterelectrode 114 and the working electrode 116. Accordingly, the referenceelectrode 118 may be configured to monitor the cell potential of theelectrochemical cell 102.

The reference electrode 118 may include nickel, nickel/nickel oxide,glassy carbon, silver/silver chloride, one or more platinum groupmetals, one or more precious metals (e.g., gold), or combinationsthereof. In some embodiments, the reference electrode 118 comprisesglassy carbon. In other embodiments, the reference electrode 118comprises nickel, nickel oxide, or a combination thereof. In yet otherembodiments, the reference electrode 118 comprises silver/silverchloride.

A potentiostat 124 may be electrically coupled to each of the counterelectrode 114, the working electrode 116, and the reference electrode118. The potentiostat 124 may be configured to measure and/or provide anelectric potential between the counter electrode 114 and the workingelectrode 116. The difference between the electric potential of thecounter electrode 114 and the electric potential of the workingelectrode 116 may be referred to as a cell potential of theelectrochemical cell 102.

The system 100 may be configured to reduce one or more metal oxides to asubstantially pure metal (e.g., a metal in a substantially unoxidizedstate) or a metal alloy. In some such embodiments, the working electrode116 includes at least one oxide (e.g., at least one metal oxide) to bereduced in the electrochemical cell 102.

The working electrode 116 may be in electrical communication with abasket 120 configured to carry one or more metals to be reduced in theelectrochemical cell 102. The basket 120 may comprise nickel, cobalt,iron, molybdenum, stainless steel, alloys of nickel and copper, alloysof nickel, chromium, iron, and molybdenum, alloys of nickel, iron, andmolybdenum, another material, or combinations thereof. In someembodiments, the basket 120 comprises nickel. In other embodiments, theelectrochemical cell 102 does not include the basket 120 and the workingelectrode 116 comprises the metal oxide or a combination of metal oxidesto be electrolytically reduced in the electrochemical cell 102. Statedanother way, in some embodiments, the working electrode 116 comprisesone or more metal oxides that are reduced to a metal (e.g., asubstantially pure metal or a metal alloy) in the electrochemical cell102. In some embodiments, the working electrode 116 consists essentiallyof the metal oxide, which may comprise one or more metals to be reduced.

At least one of the working electrode 116 and the metal in the basket120 may comprise a metal oxide. The metal oxide may comprise atransition metal oxide (such as a refractory metal oxide (e.g., titaniumoxide, (TiO), titanium dioxide (TiO₂), zirconium oxide (ZrO₂), hafniumoxide (HfO₂), vanadium oxide (V₂O₅), niobium oxide (NbO₂, Nb₂O₅),tantalum pentoxide (Ta₂O₅), chromium oxide (CrO, Cr₂O₃, etc.), manganeseoxide (MnO), nickel oxide (NiO), molybdenum oxide (MoO₃), tungsten oxide(WO₃, WO₂), ruthenium oxide (RuO₂), osmium oxide (OsO₂, OsO₄), rhodiumoxide (Rh₂O₃), iridium oxide (IrO₂)), iron oxide (Fe₂O₃, Fe₃O₄, etc.),cobalt oxide (CoO, Co₂O₃, Co₃O₄), nickel oxide (NiO, Ni₂O₃), non-metaloxides (e.g., silicon dioxide (SiO₂)), a lanthanide oxide (e.g.,lanthanum oxide (La₂O₃), cerium oxide (CeO₂), neodymium oxide (Nd₂O₃),samarium oxide (Sm₂O₃), dysprosium oxide (Dy₂O₃), another oxide of alanthanide element), an actinide oxide (e.g., actinium oxide (Ac₂O₃),thorium oxide (ThO₂), uranium oxide (e.g., UO₂), an oxide of anotheractinide element), or combinations thereof. In some embodiments, themetal oxide comprises an unirradiated nuclear fuel, such as enricheduranium oxide. In other embodiments, the metal oxide comprises a spentnuclear fuel, such as spent uranium oxide (e.g., UO₂, U₃O₈, or acombination thereof). In some embodiments, the metal oxide comprises anoxide of more than one metal. Reduction of such oxides may form a metalalloy comprising the constituent metals of the metal oxides. In someembodiments, the metal oxide is disposed in the basket 120 and inelectrical communication with the working electrode 116. In otherembodiments, the working electrode 116 consists essentially of the metaloxide.

The molten salt electrolyte 110 may include a material formulated andconfigured to facilitate reduction of the metal oxides. In someembodiments, the molten salt electrolyte 110 comprises an alkali halidesalt, an alkaline earth metal halide salt, an alkali oxide, an alkalineearth metal oxide, or combinations thereof. By way of nonlimitingexample, the molten salt electrolyte 110 may include lithium chloride(LiCl), lithium oxide (Li₂O), sodium chloride (NaCl), calcium chloride(CaCl₂), calcium oxide (CaO), lithium bromide (LiBr), potassium bromide(KBr), cesium bromide (CsBr), calcium bromide (CaBr₂), potassiumchloride (KCl), potassium bromide (KBr), strontium chloride (SrCl₂),strontium bromide (SrBr₂), or combinations thereof. In some embodiments,the molten salt electrolyte 110 comprises a eutectic mixture of sodiumchloride and potassium chloride, and may further include calcium oxide.

In some embodiments, the molten salt electrolyte 110 comprises lithiumchloride and lithium oxide (LiCl—Li₂O). In some such embodiments, thelithium oxide constitutes between about 1.0 weight percent (wt. %) andabout 5.0 weight percent of the molten salt electrolyte 110, such asbetween about 1.0 weight percent and about 2.0 weight percent, betweenabout 2.0 weight percent and about 3.0 weight percent, or between about3.0 weight percent and about 5.0 weight percent of the molten saltelectrolyte 110. The lithium chloride may constitute a remainder of themolten salt electrolyte 110. In some embodiments, the lithium oxideconstitutes about 1.0 weight percent of the molten salt electrolyte 110.In other embodiments, the lithium oxide constitutes about 5.0 weightpercent of the molten salt electrolyte 110. In some embodiments, themolten salt electrolyte 110 comprises lithium chloride and lithium oxideand the metal oxide comprises uranium oxide. In other embodiments, themetal oxide comprises one or more of manganese oxide, nickel oxide, andtitanium monoxide.

In other embodiments, the molten salt electrolyte 110 comprises calciumchloride and calcium oxide (CaCl₂—CaO). In some such embodiments, thecalcium oxide constitutes between about 1.0 weight percent and about 5.0weight percent of the molten salt electrolyte 110, such as between about1.0 weight percent and about 2.0 weight percent, between about 2.0weight percent and about 3.0 weight percent, or between about 3.0 weightpercent and about 5.0 weight percent of the molten salt electrolyte 110.The calcium chloride may constitute a remainder of the molten saltelectrolyte 110. In some embodiments, the calcium oxide constitutesabout 1.0 weight percent of the molten salt electrolyte 110. In otherembodiments, the calcium oxide constitutes about 5.0 weight percent ofthe molten salt electrolyte 110. In some embodiments, the molten saltelectrolyte 110 comprises calcium chloride and calcium oxide and themetal oxide comprises tantalum pentoxide, titanium oxide, a lanthanideoxide, an actinide oxide, or combinations thereof.

The molten salt electrolyte 110 may be maintained at a temperature suchthat the molten salt electrolyte 110 is and remains in a molten state.In other words, the temperature of the molten salt electrolyte 110 maybe maintained at or above a melting temperature of the molten saltelectrolyte 110. By way of nonlimiting example, where the molten saltelectrolyte 110 comprises lithium chloride and lithium oxide, thetemperature of the molten salt electrolyte 110 may be between about 650°C. and about 700° C. Where the molten salt electrolyte 110 comprisescalcium chloride and calcium oxide, the temperature of the molten saltelectrolyte 110 may be between about 800° C. and about 950° C. Where themolten sale electrolyte 110 comprises sodium chloride and calciumchloride, the temperature thereof may be maintained between about 550°C. and about 950° C. However, the disclosure is not so limited and thetemperature of the molten salt electrolyte 110 may be different thanthose described above.

The molten salt electrolyte 110 may facilitate reduction of the metaloxide. In some embodiments, the metal oxide may be reduced at thecathode 116, according to Equation (1) below:

M_(y)O_(x)(s)+ze ⁻ →yM+z/xO²⁻  (1),

wherein M is a metal (e.g., a transition metal, a lanthanide, anactinide, etc.), M_(y)O_(x) is the metal oxide, x is the stoichiometricamount of oxygen for the particular metal oxide, y is the stoichiometricamount of the metal in the metal oxide, and z is the stoichiometricamount of electrons for balancing the chemical reaction. The electronsare provided in the electrochemical cell 102 by provision of current tothe working electrode 116, such as through the potentiostat 124.

The oxide ions generated at the working electrode 116 may be transportedfrom the working electrode 116 to the counter electrode 114 responsiveto exposure to the applied electrical field (i.e., a polarizationbetween the counter electrode 114 and the working electrode 116,provided by the potentiostat 124). The oxide ions may be oxidized at thecounter electrode 114 according to Equation (2) below:

2O²⁻ΔO₂(g)+4e ⁻  (2).

The oxygen gas generated at the counter electrode 114 may be evolved atthe counter electrode 114. The electrons may be returned to the workingelectrode 116 surface.

In use and operation, the metal oxide may be disposed in theelectrochemical cell 102 and in contact with the molten salt electrolyte110. An electric potential may be applied between the counter electrode114 and the working electrode 116, providing a polarization field and adriving force for moving oxide ions dissolved from the metal oxide atthe working electrode 116 to the counter electrode 114, facilitatingreduction of the metal oxide at the working electrode 116.

As described above, upon depositing the electrons at the counterelectrode 114, the oxide anions may evolve as oxygen gas at the counterelectrode 114. The counter electrode 114 may be formulated andconfigured to be substantially inert to the molten salt electrolyte 110,the oxide ions, and the oxygen gas. In addition, the counter electrode114 may be substantially inert to gases that may evolve from the metaloxide. For example, where the metal oxide comprises a spent nuclearfuel, the metal oxide may include fission byproducts, such as selenium,tellurium, or iodine. The counter electrode 114 may be substantiallyinert to such gases. Accordingly, the counter electrode 114 may includea material formulated and configured to be substantially inert in theelectrochemical cell 102 at operating conditions thereof.

The counter electrode 114 may include a platinum group metal (PGM), aprecious metal (e.g., silver or gold), or a combination thereof. By wayof nonlimiting example, the counter electrode 114 may include ruthenium,osmium, rhodium, iridium, palladium, platinum, silver, gold, orcombinations thereof. In some embodiments, the counter electrode 114includes a ternary compound including a platinum group metal, oxygen,and one of an alkali metal and an alkaline earth metal. The ternarycompound may have a general formula of M₁M₂O_(x), wherein M₁ is one ormore of lithium, sodium, potassium, magnesium, calcium, or strontium, M₂is a platinum group metal such as ruthenium, osmium, rhodium, iridium,palladium, platinum, and combinations thereof, and x is an integer, suchas 2, 3 or 4, depending on M₁. In other embodiments, M₂ may be tin ormanganese. By way of nonlimiting example, the counter electrode 114 maycomprise lithium iridate (Li₂IrO₃), lithium ruthenate (Li₂RuO₃), lithiumrhodates (LiRhO₂, LiRhO₃), a lithium tin oxygen compound (e.g.,Li₂SnO₃), a lithium manganese oxygen compound (Li₂MnO₃), calciumruthenate (CaRuO₃), strontium ruthenium ternary compounds (SrRuO₃,Sr₂RuO₃, Sr₂RuO₄), calcium iridate (CaIrO₃), strontium iridate (SrIrO₃,SrIrO₄, Sr₂IrO₄), calcium platinate (CaPtO₃), strontium platinate(SrPtO₄), magnesium ruthenate (MgRuO₄), magnesium iridate (MgIrO₄),sodium ruthenate (Na₂RuO₄), sodium iridate (Na₂IrO₃), potassium iridate(K₂IrO₃), and potassium ruthenate (K₂RuO₄), and combinations thereof. Insome embodiments, where the counter electrode 114 comprises a ternarycompound, M₁ may be selected to comprise a material of the molten saltelectrolyte 110. By way of nonlimiting example, where the molten saltelectrolyte comprises lithium (e.g., LiCl/Li₂O), M₁ may be selected tobe lithium. Similarly, where the molten salt electrolyte 110 comprisescalcium (e.g., CaCl₂/CaO), M₁ may be selected to be calcium.

In some embodiments, the counter electrode 114 comprises a monolithicmaterial. In some such embodiments, the counter electrode 114 comprisesa monolithic metal, such as monolithic iridium, monolithic ruthenium,monolithic osmium, monolithic rhodium, monolithic palladium, ormonolithic platinum. In other embodiments, the counter electrode 114comprises a monolithic structure of one of lithium iridate, lithiumruthenate, a lithium rhodate, a lithium tin oxygen compound, a lithiummanganese compound, calcium ruthenate, a strontium ruthenium ternarycompound, calcium iridate, strontium iridate, calcium platinate,strontium platinate, magnesium ruthenate, magnesium iridate, sodiumruthenate, sodium iridate, potassium iridate, potassium ruthenate, orcombinations thereof.

As illustrated in FIG. 1 , the counter electrode 114 may comprise amonolithic body comprising a substantially uniform composition. In otherembodiments, the counter electrode 114 may comprise a base material(e.g., a substrate) coated with counter electrode material formulatedand configured to be substantially inert in the electrochemical cell102. FIG. 2 is a simplified cross-sectional view of a counter electrode114′, in accordance with some embodiments of the disclosure. The counterelectrode 114′ may comprise a substrate (e.g., a core) 130 and a coatingmaterial 132. The coating material 132 may include one or more of thematerials described above with reference to the counter electrode 114 ofFIG. 1 . For example, the coating material 132 may comprise ruthenium,osmium, rhodium, iridium, palladium, platinum, silver, gold, lithiumiridate, lithium ruthenate, a lithium rhodate, a lithium tin oxygencompound, a lithium manganese compound, calcium ruthenate, a strontiumruthenium ternary compound, calcium iridate, strontium iridate, calciumplatinate, strontium platinate, magnesium ruthenate, magnesium iridate,sodium ruthenate, sodium iridate, potassium iridate, potassiumruthenate, or combinations thereof.

The coating material 132 may be substantially uniform over surfaces ofthe substrate 130. The coating material 132 may have a thickness Tbetween about 1.0 mm and about 7.0 mm, such as between about 1.0 mm andabout 3.0 mm, between about 3.0 mm and about 5.0 mm, or between about5.0 mm and about 7.0 mm. In some embodiments, the thickness T is betweenabout 3.0 mm and about 5.0 mm.

The substrate 130 may comprise a material that is different than thecoating material 132. The substrate 130 may comprise a metal or anon-metal. The substrate 130 may be selected to exhibit a coefficient ofthermal expansion substantially similar to a coefficient of thermalexpansion of the coating material 132. By way of nonlimiting example,the substrate 130 may comprise graphite (e.g., high density graphite),nickel, molybdenum, tantalum, chromium, tungsten, titanium, or anothermaterial. In some embodiments, the substrate 130 comprises high densitygraphite.

Referring to FIG. 3 , an electrochemical cell 300 for forming thecounter electrode 114′ (FIG. 2 ) is illustrated. The electrochemicalcell 300 includes an anode 302 and a cathode 304 immersed in a moltensalt electrolyte 306. A power source 308, such as a direct current powersource, may be operably coupled to the anode 302 and the cathode 304 forproviding an electric potential between the anode 302 and the cathode304 and a driving force for electrochemical reactions that may occurwithin the electrochemical cell 300.

The electrochemical cell 300 may be configured as an electroplatingelectrochemical cell, wherein one or more components of the molten saltelectrolyte 306 comprises a material to be deposited on the cathode 304.The molten salt electrolyte 306 may be formulated and configured toinclude at least one component of the coating material 132 (FIG. 2 ) ofthe counter electrode 114′ (FIG. 2 ). The molten salt electrolyte 306may include, for example, a fluoride salt, chloride salt, a bromidesalt, or another salt of one or more of ruthenium, osmium, rhodium,iridium, palladium, platinum, gold, and silver. By way of nonlimitingexample, the molten salt electrolyte 306 may include ruthenium fluoride(RuF₃, RuF₄, RuF₆, (RuF₅)₄), ruthenium chloride (RuCl₂, RuCl₃),ruthenium bromide (RuBr₃), osmium fluoride (OsF₆), osmium chloride(OsCl₃, OsCl₄), osmium bromide (OsBr₃, OsBr₄), rhodium chloride (RhCl₃),rhodium (RhBr₃), iridium fluoride (IrF₃, IrF₄, IrF₆), iridium chloride(IrCl₂, IrCl₃, IrCl₄), iridium bromide (IrBr₃, IrBr₄), palladiumfluoride (PdF₂, PdF₄), palladium chloride (PdCl₂), palladium bromide(PdBr₂), platinum fluoride (PtF₄, (PtF₅)₄, PtF₆), platinum chloride(PtCl₂, PtCl₃, PtCl₄), platinum bromide (PtBr₂, PtBr₃, PtBr₄), silverfluoride (AgF), silver chloride (AgCl), silver bromide (AgBr), goldfluoride (AuF₃, AuF₅), gold chloride (AuCl, AuCl₃), gold bromide (AuBr,(AuBr₃)₂), potassium hexachlorosmate (K₂OsCl₆), H₂PtCl₆, (NH₄)₂PtCl₆,Pt(NO₂)₂(NH₃)₂, K₂Pt(NO₂)₄, Na₆Pt(SO₃)₄, Na₆Pt(SO₃)₄, or combinationsthereof.

The anode 302 may include graphite, glassy carbon, nickel, cobalt,molybdenum, tantalum, tungsten, a platinum group metal, or anothermaterial. In some embodiments, the anode 302 comprises graphite. In someembodiments, where the anode 302 comprises molybdenum, tantalum,tungsten, or combinations thereof, the electrolyte 306 may comprise abromide salt including at least one platinum group metal. In otherembodiments, where the electrolyte 306 comprises dissolved platinumgroup metals dissolved in a chloride salt, a fluoride salt, or acombination thereof, the anode 302 may comprise graphite or glassycarbon. In further embodiments, where the anode 302 comprises a platinumgroup metal, the electrolyte 306 may comprise a bromide salt, a fluoridesalt, a chloride salt, or combinations thereof.

The cathode 304 may include any material on which the coating material132 (FIG. 2 ) may be formed (i.e., a suitable substrate 130 (FIG. 2 )).Stated another way, the cathode 304 may correspond to the substrate 130(FIG. 2 ) and may include any of the materials described above withreference to the substrate 130. By way of nonlimiting example, thecathode 304 may include graphite (e.g., high density graphite), nickel,molybdenum, tantalum, titanium, chromium, tungsten, another material, orcombinations thereof.

Application of an electric potential between the anode 302 and thecathode 304 may cause dissolved ions (one or more of dissolvedruthenium, osmium, rhodium, iridium, palladium, platinum, silver, andgold ions) to move to the cathode 304, where such ions may be depositedto form a coating (e.g., the coating material 132 (FIG. 2 )). After asufficient duration, the cathode 304 may be coated to a desiredthickness and the cathode 304 is removed from the electrochemical cell300.

In some embodiments, after removing the cathode 304 from theelectrochemical cell 300, the cathode 304 is annealed, such as byexposing the cathode 304 to a temperature greater than about 1,000° C.,greater than about 1,100° C., greater than about 1,200° C., greater thanabout 1,300° C., greater than about 1,400° C., greater than about 1,500°C., greater than about 1,600° C., greater than about 1,700° C., greaterthan about 1,800° C., greater than about 1,900° C., or even greater thanabout 2,000° C. In some embodiments, the annealing temperature may beless than a melting temperature of the coating material 132 (e.g., lessthan about 75% of the melting temperature of the coating material 132 indegrees Celsius).

The cathode 304 may be exposed to the annealing temperature for aduration between about 2 hours and about 7 days, such as between about 2hours and about 6 hours, between about 6 hours and about 12 hours,between about 12 hours and about 1 day, between about 1 day and about 2days, between about 2 days and about 3 days, between about 3 days andabout 5 days, or between about 5 days and about 7 days.

The annealed cathode 304 may be used as a counter electrode (e.g., thecounter electrode 114′ (FIG. 2 )) in the electrochemical cell 102 (FIG.1 ) for reducing metal oxides at the cathode 116 (FIG. 1 ).

Although the counter electrode 114′ (FIG. 2 ) has been described asbeing formed in the electroplating electrochemical cell 300 (FIG. 3 ),the disclosure is not so limited. In other embodiments, the counterelectrode 114′ may be formed by depositing the coating 132 (FIG. 2 ) onthe substrate 130 (FIG. 1 ). By way of nonlimiting example, the coating132 may be formed on the substrate 130 by chemical vapor deposition(CVD), physical vapor deposition (PVD), atomic layer deposition (ALD),plasma-enhanced chemical vapor deposition (PECVD), or another depositionmethod.

FIG. 4 is a simplified flow diagram of a method 400 of reducing one ormore metal oxides in an electrochemical cell, in accordance withembodiments of the disclosure. The method 400 includes act 402,including disposing one or more metal oxides in an electrochemical cell;act 404 including applying electric potential between a counterelectrode and a working electrode of the electrochemical cell; and act406 including reducing metal oxides at the cathode to form asubstantially pure metal or a metal alloy.

Act 402 may include disposing one or more metal oxides in anelectrochemical cell. The electrochemical cell may be substantiallysimilar to the electrochemical cell 102 described above with referenceto FIG. 1 . The metal oxide may comprise a metal oxide to be reduced,such as for example, uranium oxide, titanium monoxide, titanium dioxide,magnesium oxide, zirconium oxide, hafnium oxide, vanadium oxide, niobiumoxide, tantalum pentoxide, chromium oxide, molybdenum oxide, tungstenoxide, ruthenium oxide, osmium oxide, rhodium oxide, iridium oxide, ironoxide, cobalt oxide, nickel oxide, silicon oxide, a lanthanide oxide, anactinide oxide, or combinations thereof. In some embodiments, the metaloxide comprises the working electrode of the electrochemical cell. Insome such embodiments, the working electrode consists essentially of themetal oxide. In other embodiments, the metal oxide is directly securedto the working electrode, such as in a metal basket. In some suchembodiments, the basket may comprise nickel, stainless steel, or anothermaterial.

The electrochemical cell may include a molten salt electrolyte that maybe substantially similar to those described above with reference to FIG.1 . For example, the molten salt electrolyte may comprise a mixture oflithium chloride and lithium oxide, a mixture of calcium chloride andcalcium oxide, a eutectic mixture of sodium chloride and calciumchloride, a eutectic mixture of sodium chloride and potassium chloride,or combinations thereof.

The counter electrode of the electrochemical cell may be substantiallysimilar to the counter electrode materials described above withreference to FIG. 1 . In some embodiments, the counter electrodecomprises a platinum group metal.

Act 404 may include applying an electric potential between the counterelectrode and the working electrode of the electrochemical cell. Theelectric potential may be selected based on the composition of themolten salt electrolyte. By way of nonlimiting example, where the moltensalt electrolyte comprises lithium chloride and lithium oxide, theelectric potential (also referred to as the “cathodic potential”) may bebetween about 0.1 V and about 3.1 V, such as between about 0.1 V andabout 1.5 V, between about 1.5 V and about 2.0 V, between about 2.0 Vand about 2.5 V, between about 2.5 V and about 2.7 V, between about 2.7V and about 2.9 V, or between about 2.9 V and about 3.1 V. Where themolten salt electrolyte comprises calcium chloride and calcium oxide,the electric (cathodic) potential may be between about 0.1 V and about3.2 V, such as between about 0.1 V and about 2.0 V, between about 2.0 Vand about 2.5 V, between about 2.5 V and about 2.7 V, between about 2.7V and about 2.9 V, or between about 2.9 V and about 3.2 V. Where themolten salt electrolyte comprises eutectic mixture of sodium chlorideand potassium chloride, the electric potential may be between about 0.1V and about 3.2 V, such as between about 0.1 V and about 2.0 V, betweenabout 2.0 V and about 2.5 V, between about 2.5 V and about 2.7 V,between about 2.7 V and about 2.9 V, or between about 2.9 V and about3.2 V.

Act 404 may further include maintaining a temperature of theelectrochemical cell at or above a melting temperature of the moltensalt electrolyte such that the molten salt electrolyte remains in amolten state. By way of nonlimiting example, the molten salt electrolytemay be maintained at a temperature between about 640° C. and about 680°C., such as between about 640° C. and about 660° C. or between about660° C. and about 680° C. where the molten salt electrolyte comprisesLiCl—Li₂O. In some such embodiments, the molten salt electrolyte may bemaintained at a temperature of about 650° C. In embodiments where themolten salt electrolyte comprises CaCl₂—CaO, the molten salt electrolytemay be maintained at a temperature between about 800° C. and about 950°C., such as between about 800° C. and about 850° C., between about 850°C. and about 875° C., between about 875° C. and about 900° C., orbetween about 900° C. and about 950° C. Where the molten saltelectrolyte comprises sodium chloride and calcium chloride, the moltensalt electrolyte may be maintained at a temperature between about 550°C. and about 950° C., such as between about 550° C. and about 600° C.,between about 600° C. and about 700° C., between about 700° C. and about800° C., between about 800° C. and about 900° C., or between about 950°C. and about 950° C. Of course, it is contemplated that the temperatureof the molten salt electrolyte may be maintained at a temperature otherthan the temperatures described above, depending on the composition ofthe molten salt electrolyte and desired operating conditions.

Act 406 may include reducing metal oxides at the working electrode toform a substantially pure metal or metal alloy. In some embodiments, themetal may exhibit substantially the same composition as the metal oxideof the working electrode, except that the metal or metal alloy may besubstantially free of oxygen.

In some embodiments, the pressure within the electrochemical cell may beequal to about ambient pressure (i.e., atmospheric pressure). However,the disclosure is not so limited and the atmospheric pressure of theelectrochemical cell may be different than those described above.

Accordingly, in some embodiments, one or more metal oxides may bedisposed on or proximate to the working electrode 116 (FIG. 1 ) of theelectrochemical cell 102 (FIG. 1 ). The counter electrode 114 (FIG. 1 )may be selected to comprise a material that is compatible with themolten salt electrolyte 110 (FIG. 1 ) and does not substantially corrodeor wear (e.g., thin) responsive to exposure to the molten saltelectrolyte 110.

Without wishing to be bound by any particular theory, it is believedthat forming the counter electrode 114 from the materials describedherein facilitates operating the electrochemical cell 102 and reducingthe metal oxides without substantially degrading or otherwise consumingthe counter electrode 114. The counter electrode materials describedherein may not substantially react with the molten salt electrolyte,oxide ions, or evolved reactive gases (e.g., selenium, tellurium, andiodine) at operating conditions of the electrochemical cell 102.Accordingly, such materials may maintain structural integrity duringoperation of the electrochemical cell 102. By way of comparison,conventional counter electrode materials (e.g., platinum, graphite,etc.) may react with at least one of one or more constituents of themolten salt electrolyte, oxide ions, or evolved gases, degrading theconventional counter electrode material.

Advantageously, since the electrochemical cell 102 (FIG. 1 ) isconfigured as a direct oxide reduction cell, metal ions of the metaloxide are not substantially dissolved in the molten salt electrolyte 110(FIG. 1 ). Accordingly, the metal oxide is reduced to the metal withoutmetal ions dissolving in the molten salt electrolyte 110. Since themetal oxide is not dissolved in the molten salt electrolyte 110, in someembodiments, operation of the electrochemical cell 102 may consume lesspower than in other conventional electrochemical cells wherein the metaloxide being reduced is dissolved in the molten salt electrolyte. Inaddition, since the metal oxide is not dissolved, the size and shape ofthe metal formed may correspond to the size and shape of the metal oxidethat is reduced in the electrochemical cell 102.

Without wishing to be bound by any particular theory, it is believedthat the counter electrode 114 materials described herein aresubstantially inert in the electrochemical cell 102 and are notsubstantially consumed because they do not react with or form complexesor compounds with the molten salt electrolyte 110. By way of nonlimitingexample, iridium anode materials do not react in lithiumchloride/lithium oxide molten salt electrolytes to form lithium iridatesince the formation temperature of lithium iridate is higher than about750° C. in such molten salts. Similarly, ruthenium counter electrodematerials do not react with such molten salt electrolytes to formlithium ruthenate since the formation temperature of lithium ruthenateis higher than 650° C. Similarly, lithium rhodates, includingdioxorhodates (e.g., Li₂RhO₂) and trioxorhodates (e.g., Li₂RhO₃), onlyform at a temperature greater than about 825° C. Accordingly, suchmaterials can be used in the electrochemical cell 102 (FIG. 1 ) (e.g.,an electrochemical cell with a LiCl—Li₂O molten salt electrolyte havinga temperature less than about 825° C.) since they do not form and reactwith the molten salt electrolyte. In addition, since lithium iridate,lithium ruthenate, and lithium rhodates are not formed at the operatingconditions of the electrochemical cell, the counter electrode maycomprise lithium iridate, lithium ruthenate, one or more lithiumrhodates, or combinations thereof and may be substantially inert in theelectrochemical cell.

As another example, calcium ruthenate, strontium ruthenium ternarycompounds, calcium iridate, strontium iridate, calcium platinate, andstrontium platinate form at high gas pressures (such as at pressuresbetween about 1.0 GPa and about 5.0 GPa) and do not substantially format operating conditions of an electrochemical cell with either a calciumchloride-calcium oxide or a sodium chloride-calcium chloride molten saltelectrolyte. Accordingly, counter electrodes comprising one or more ofthe materials described above in such molten salt electrolytes may besubstantially inert under operating conditions of the electrochemicalcell. In addition, counter electrode materials comprising calciumruthenate, strontium ruthenium ternary compounds, calcium iridate,strontium iridate, calcium platinate, and strontium platinate may besubstantially inert in the electrochemical cell, since they form at gaspressures above operating pressures of the electrochemical cell 102 anddo not react with the molten salt electrolyte 110 at operatingconditions of the electrochemical cell 102.

EXAMPLES Example 1

An electrolyte comprising 200 g of high purity anhydrous calciumchloride was prepared. Calcium oxide was added to the electrolyte suchthat the calcium oxide constituted about 1.0 weight percent of theelectrolyte. The electrolyte was melted in an alumina/nickel crucibleinside a glove box under an argon atmosphere having less than 0.1 ppmmoisture and oxygen. A first monolithic counter electrode (anode)comprising a monolithic rod of iridium and a second counter electrodecomprising a monolithic rod of ruthenium, a working electrode (cathode)comprising tantalum pentoxide pellets in a basket, and a referenceelectrode comprising glassy carbon were cleaned in an ultrasonic bath,oven dried, and disposed in the electrochemical cell. The workingelectrode included about 18.1 weight percent oxygen. The first counterelectrode, the second counter electrode, and reference electrode rodshad a diameter of about 3.0 mm and a length of about 100 mm. The workingelectrode was prepared by sintering oxides of tantalum in air or in areducing atmosphere to form sintered pellets. The sintered pellets werecylindrical in shape and had a diameter between about 13.0 mm and about15.0 mm and a length between about 1.0 mm and about 5.0 mm. The firstcounter electrode, the second counter electrode, the working electrode,and reference electrode were sheathed in high purity alumina tubes.

The electrolyte was heated in a furnace to melt the electrolyte and forma molten salt electrolyte. The molten salt electrolyte was maintained ata temperature between about 800° C. and about 950° C. The first counterelectrode, the second counter electrode, the working electrode, and thereference electrode were disposed in the electrochemical cell such thatthe lower portions of the respective electrodes were in contact with themolten salt electrolyte. The cell voltage was controlled between about2.5 V and about 3.0 V. The current was measured as a function of time,as illustrated in the graph of FIG. 5 . The electrochemical cell wasoperated inside an argon atmosphere in a glove box.

As illustrated in FIG. 5 , the initial current measured exhibited asharp rise before declining over time. The initial peak in the currentis due to the non-conducting nature of the metal oxide and the reductionin current correlates to the reduction of the metal oxide to aconducting metal.

The residual oxygen content of the metal oxide was measured with a LECOanalyzer. The metal oxide was reduced by about 98.9%. In other words,after the electrochemical reaction, the metal oxide had less than about2,000 ppm oxygen, indicating a reduction in oxygen content of about98.9%. Stated another way, the metal oxide was reduced and includedabout 0.2 weight percent oxygen.

The first counter electrode and second counter electrode were visuallyinspected to record the occurrence of any possible mechanicaldegradation such as cracking, thinning, corrosion, erosion, or necking.No mechanical degradation was observed in the anodes.

Example 2

A first counter electrode (anode) comprising platinum was disposed in anelectrochemical cell including a molten salt electrolyte comprisinglithium chloride and lithium oxide. The electrochemical cell includeddepleted uranium oxide disposed in a working electrode (cathode) basketwhich was in contact with the cathode lead. A second iridium counterelectrode was disposed in the same electrochemical cell. Theelectrochemical cell was in an argon atmosphere in a glove box.

A current was applied between and the single cathode and each of theplatinum counter electrode and the iridium counter electrode and themetal oxide was reduced in the electrochemical cell. After the metaloxide was substantially reduced, both the platinum counter electrode andthe iridium counter electrode were removed and analyzed for mechanicaldegradation and thinning. The platinum counter electrode exhibited areduction in diameter of about 17 percent. The iridium counter electrodeexhibited a reduction in diameter of less than about 1 percent.Accordingly, the iridium counter electrode exhibited substantially lessthinning than the platinum counter electrode.

Example 3

A first counter electrode (anode) comprising monolithic iridium and asecond counter electrode (anode) comprising monolithic ruthenium weredisposed in an electrochemical cell including a molten salt electrolytecomprising lithium chloride and lithium oxide. The first counterelectrode and the second counter electrode comprised rods with adiameter of 3.0 mm and a length of about 100 mm.

The molten salt electrolyte comprised about 1.0 weight percent lithiumoxide. The temperature of the molten salt electrolyte was about 650° C.The electrochemical cell included about 100 grams of unirradiateddepleted uranium oxide (UO₂) disposed in a stainless steel basket whichwas in contact with the cathode lead. The electrochemical cell was in anargon atmosphere in a glove box.

A current was applied between the single cathode and each of the firstcounter electrode (monolithic iridium) and the second counter electrode(monolithic ruthenium) and the uranium oxide was reduced in theelectrochemical cell. After about 80 hours, both the first counterelectrode and the second counter electrode were removed and analyzed formechanical degradation and thinning. No perceptible mechanicaldegradation was observed in either of the first counter electrode or thesecond counter electrode. Accordingly, the first counter electrode andthe second counter electrode exhibited substantially less thinning thana platinum electrode (such as the platinum electrode of Example 2).

Example 4

An electrochemical cell comprising a molten salt electrolyte including amixture of LiCl and about 1.0 weight percent Li₂O, about 0.1 weightpercent Na₂Se, about 0.1 weight percent Li₂Te, and about 0.1 weightpercent LiI was prepared. The working electrode (cathode) of theelectrochemical cell included pellets and/or chunks of titaniummonoxide, manganese oxide, and nickel oxide (NiO) packed into astainless steel basket. The stainless steel basket was in electricalcommunication with the working electrode through a stainless steel wireused as the working electrode current collector. Three counter electrode(anodes) were disposed in the electrochemical cell: one counterelectrode comprising monolithic iridium; one counter electrodecomprising monolithic ruthenium; and one counter electrode comprisingmonolithic platinum. The counter electrodes each had a diameter of about3.0 mm and a length of about 100.0 mm. The electrochemical cell wasoperated to reduce the oxides in the stainless steel basket. Selenium,tellurium, and iodine gases evolved during operation of theelectrochemical cell. After the metal oxides in the stainless steelbasket were reduced, the counter electrodes were inspected. The platinumcounter electrode exhibited significant thinning (e.g., necking), whilethe ruthenium counter electrode and the iridium counter electrode didnot exhibit any detectable material loss.

While embodiments of the disclosure may be susceptible to variousmodifications and alternative forms, specific embodiments have beenshown by way of example in the drawings and have been described indetail herein. However, it should be understood that the disclosure isnot limited to the particular forms disclosed. Rather, the disclosureencompasses all modifications, variations, combinations, andalternatives falling within the scope of the disclosure as defined bythe following appended claims and their legal equivalents.

1. A method of direct oxide reduction, the method comprising: disposinga molten salt electrolyte in an electrochemical cell; at least partiallyimmersing an anode in the molten salt electrolyte, the anode comprisingmonolithic iridium or monolithic ruthenium; disposing a cathode in theelectrochemical cell, comprising disposing at least one metal oxide incontact with the molten salt electrolyte; and applying an electricpotential between the anode and the cathode.
 2. The method of claim 1,wherein disposing the cathode in the electrochemical cell furthercomprises disposing, in the electrochemical cell, an electrodeconsisting essentially of the at least one metal oxide.
 3. The method ofclaim 1, wherein: disposing a molten salt electrolyte in theelectrochemical cell comprises disposing, in the electrochemical cell, alithium chloride/lithium oxide (LiCl—Li₂O) molten salt electrolyte; andfurther comprising, at least while the at least one metal oxide isdisposed in contact with the molten salt electrolyte, maintaining atemperature of the electrochemical cell at or above a meltingtemperature of the molten salt electrolyte but below 650° C.
 4. Themethod of claim 1, wherein: disposing a molten salt electrolyte in theelectrochemical cell comprises disposing, in the electrochemical cell, acalcium chloride-calcium oxide (CaCl₂—CaO) molten salt electrolyte or asodium chloride-calcium chloride (NaCl—CaCl₂) molten salt electrolyte;and further comprising, at least while the at least one metal oxide isdisposed in contact with the molten salt electrolyte, operating theelectrochemical cell at a pressure below about 1.0 GPa.
 5. A method ofreducing nuclear fuels comprising uranium oxide, the method comprising:providing an electrochemical cell comprising a working electrode, and acounter electrode comprising one or more materials selected from thegroup consisting of osmium, ruthenium, rhodium, iridium, palladium,platinum, silver, gold, lithium iridate, lithium ruthenate, lithiumrhodates, a lithium tin oxygen compound, a lithium manganese compound,strontium ruthenium ternary compounds, calcium iridate, strontiumiridate, calcium platinate, strontium platinate, magnesium ruthenate,magnesium iridate, sodium ruthenate, sodium iridate, potassium iridate,and potassium ruthenate; disposing a uranium oxide material on orproximate the working electrode; exposing the uranium oxide to a moltensalt electrolyte comprising at least one of lithium chloride, lithiumoxide; calcium chloride, calcium oxide, and sodium chloride; and passingan electric current between the counter electrode and the workingelectrode to reduce the uranium oxide material.
 6. The method of claim5, further comprising selecting the counter electrode to comprise aplatinum group metal.
 7. The method of claim 5, further comprisingselecting the counter electrode to comprise a substrate materialselected from the group consisting of high density graphite, molybdenum,tantalum, titanium, nickel, chromium, tungsten, and combinationsthereof, wherein the substrate material is coated with the one or morematerials selected from the group consisting of osmium, ruthenium,rhodium, iridium, palladium, platinum, silver, gold, lithium iridate,lithium ruthenate, lithium rhodates, a lithium tin oxygen compound, a.lithium manganese compound, strontium ruthenium ternary compounds,calcium iridate, strontium iridate, calcium platinate, strontiumplatinate, magnesium ruthenate, magnesium iridate, sodium ruthenate,sodium iridate, potassium iridate, and potassium ruthenate.
 8. Themethod of claim 5, further comprising selecting the counter electrode tocomprise a substrate material coated with a coating material comprisingthe one or more materials selected from the group consisting of osmium,ruthenium, rhodium, iridium, palladium, platinum, silver, gold, lithiumiridate, lithium ruthenate, lithium rhodates, a lithium tin oxygencompound, a lithium manganese compound, strontium ruthenium ternarycompounds, calcium iridate, strontium iridate, calcium platinate,strontium platinate, magnesium ruthenate, magnesium iridate, sodiumruthenate, sodium iridate, potassium iridate, and potassium ruthenateand having a thickness between about 3.0 mm and about 5.0 mm.
 9. Themethod of claim 5, further comprising selecting the counter electrode tocomprise ruthenium.
 10. The method of claim 5, further comprisingselecting the molten salt electrolyte to comprise lithium chloride andlithium oxide.
 11. The method of claim 5, further comprising selectingthe counter electrode to comprise lithium iridate, lithium ruthenate, ora lithium rhodate.
 12. The method of claim 5, further comprisingselecting the molten salt electrolyte to comprise calcium chloride andcalcium oxide.
 13. The method of claim 5, further comprising selectingthe uranium oxide to comprise unirradiated uranium oxide.
 14. A methodof direct oxide reduction, the method comprising: forming a molten saltelectrolyte in an electrochemical cell; disposing at least one metaloxide in the electrochemical cell; disposing a counter electrodecomprising a material selected from the group consisting of osmium,ruthenium, rhodium, iridium, palladium, platinum, silver, gold, lithiumiridate, ruthenate, a lithium rhodate, a lithium tin oxygen compound, alithium manganese compound, strontium ruthenium ternary compounds,calcium iridate, strontium iridate, calcium platinate, strontiumplatinate, magnesium ruthenate, magnesium iridate, sodium ruthenate,sodium iridate, potassium iridate, and potassium ruthenate in theelectrochemical cell; and applying a current between the counterelectrode and the at least one metal oxide to reduce the at least onemetal oxide.
 15. The method of claim 14, wherein forming a molten saltelectrolyte in the electrochemical cell comprises forming the moltensalt electrolyte from one or more of at least one chloride salt and atleast one bromide salt.
 16. The method of claim 14, wherein disposing atleast one metal oxide in the electrochemical cell comprises forming aworking electrode of the electrochemical cell from at least one ofuranium oxide, tantalum pentoxide, titanium monoxide; titanium dioxide,zirconium oxide, hafnium oxide, tungsten oxide, chromium oxide,molybdenum oxide, iron oxide, nickel oxide, neodymium oxide, samariumoxide, and silicon oxide.
 17. The method of claim 14, wherein disposingat least one metal oxide in the electrochemical cell comprises forming aworking electrode of the electrochemical cell from at least one of atleast one lanthanide oxide and at least one actinide oxide.
 18. Themethod of claim 14, wherein disposing a counter electrode in theelectrochemical cell comprises disposing a counter electrode comprisinga ternary compound of a platinum group metal.
 19. The method of claim18, further comprising selecting the ternary compound to furthercomprise at least one of an alkali metal and an alkaline earth metal.20. The method of claim 14, wherein: disposing at least one metal oxidein the electrochemical cell comprises disposing more than one metaloxide in the electrochemical cell; and applying a current between thecounter electrode and the at least one metal oxide to reduce the atleast one metal oxide comprises reducing the more than one metal oxideto form a metal alloy.
 21. The method of claim 14, wherein applying acurrent between the counter electrode and the at least one metal oxidecomprises applying the current between the counter electrode and the atleast one metal oxide comprising iridium oxide and ruthenium oxide toreduce the iridium oxide and ruthenium oxide to iridium and ruthenium.